Exhaust gas heating method

ABSTRACT

A method according to the present invention of heating exhaust gas to be introduced into an exhaust gas purifying device from an internal combustion engine by heating and igniting fuel supplied into an exhaust passage from a fuel supply valve includes the steps of determining a necessity for supplying fuel into the exhaust passage, setting an amount of fuel to be supplied into the exhaust passage from the fuel supply valve, the amount being set when it is judged in the determining step that fuel needs to be supplied into the exhaust passage, checking an operating state of the engine, and controlling an air-fuel ratio of exhaust gas such that the air-fuel ratio falls within an area where amounts of both smoke and HC to be generated in the exhaust gas are low in the operating state of the engine checked in the checking step.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national phase application of InternationalApplication No. PCT/JP2012/000006, filed Jan. 4, 2012, the content ofwhich is incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a method of heating exhaust gas to beintroduced into an exhaust gas purifying device by supplying fuel intoan exhaust passage from a fuel supply valve disposed upstream of theexhaust gas purifying device in an exhaust pipe, and then by heating andigniting the fuel.

BACKGROUND ART

In recent years, coping with strict emission standards set on internalcombustion engines has lead to a necessity for facilitating theactivation of an exhaust gas purifying device at the start of itsinternal combustion engine, maintaining its active state during theoperation of the internal combustion engine, and so on. In this respect,Japanese Patent Laid-Open No. 2010-084710 and the like have proposedinternal combustion engines in which an exhaust gas heating device isinstalled upstream of an exhaust gas purifying device in an exhaustpassage. This exhaust gas heating device generates heating gas withinexhaust gas and supplies this generated heating gas into the exhaust gaspurifying device given at the downstream side to thereby facilitate theactivation of the exhaust gas purifying device and maintain its activestate. To do so, the exhaust gas heating device generally includes afuel supply valve which adds fuel into the exhaust passage, and anigniter such as a glow plug which heats and ignites the fuel to generateheating gas. Meanwhile, in the conventional exhaust gas heating devicedisclosed in Japanese Patent Laid-Open No. 2010-084710, the igniter isdisposed in proximity to the fuel supply valve. Hence, the fuel suppliedinto the exhaust passage from the fuel supply valve and the exhaust gasmay fail to be mixed sufficiently, leading to incomplete combustion ofthe ignited fuel in some cases. To solve this, Japanese Patent Laid-OpenNo. 2010-084710 proposes an idea that a receiving plate which receivesthe fuel injected from the fuel supply valve and scatters that fuelinside the exhaust passage is installed in the exhaust passage.

CITATION LIST Patent Literature

-   PTL 1: Japanese Patent Laid-Open No. 2010-084710

SUMMARY OF INVENTION Technical Problem

The fuel addition into the exhaust passage differs from the fuelinjection into the combustion chamber of the engine in that theinjection pressure of the fuel tends to be low and the pressure andtemperature thereof tend to be absolutely low as well. This retards thefuel's vaporization and causes residual droplets of the fuel within theexhaust gas. Thus, the fuel supplied into the exhaust passage may failto be well combusted depending upon the operating state of the internalcombustion engine, which in turn leads to a possibility of generating alarge amount of smoke (or soot). For example, when the exhaust gasflowing through the exhaust passage has a high oxygen concentration or ahigh exhaust gas temperature, the combustion rate of the fuel duringignition by the igniter tends to increase, thus generating a largeamount of smoke. Consequently, unburned fuel flows to the downstreamside as it is, resulting in the unburned fuel adhering to a catalyticconverter forming part of the exhaust gas purifying device and the fuelpassing therethrough, thereby possibly generating white smoke.

In the conventional exhaust gas heating device disclosed in JapanesePatent Laid-Open No. 2010-084710, the igniter is disposed in proximityto the fuel supply valve, and hence the fuel supplied into the exhaustpassage from the fuel supply valve and the exhaust gas may fail to bemixed sufficiently, leading to incomplete combustion of the ignited fuelin some cases. This in turn causes the generation of a large amount ofsmoke and clogging of the catalytic converter forming part of theexhaust gas purifying device, which is troublesome. As a result, therearises a problem that the catalytic converter needs to undergo frequentregeneration processes, deteriorating the fuel efficiency due to theconsumption of fuel for the frequent regeneration processes.

An object of the present invention is to provide a method capable ofalways suppressing the generation of smoke and HC to a small amount whenfuel is supplied into an exhaust passage from a fuel supply valvedisposed upstream of an exhaust gas purifying device in an exhaust pipe,and then by heating and igniting the fuel to heat exhaust gas to beintroduced into the exhaust gas purifying device.

Solution to Problem

A method according to the present invention of heating exhaust gas to beintroduced into an exhaust gas purifying device from an internalcombustion engine by supplying fuel into an exhaust passage from a fuelsupply valve disposed upstream of the exhaust gas purifying device in anexhaust pipe upstream, and then by heating and igniting the fuelsupplied into the exhaust passage, comprises the steps of determining anecessity for supplying fuel into the exhaust passage, setting an amountof fuel to be supplied into the exhaust passage from the fuel supplyvalve, the amount being set when it is judged in the determining stepthat fuel needs to be supplied into the exhaust passage, checking anoperating state of the internal combustion engine, and controlling anair-fuel ratio of exhaust gas to be released into the exhaust passagefrom a combustion chamber of the internal combustion engine such thatthe air-fuel ratio falls within an area where amounts of both smoke andHC to be generated in the exhaust gas flowing through the exhaustpassage are low in the operating state of the internal combustion enginechecked in the checking step.

The combustion state of the fuel in the exhaust gas heating devicevaries among a state where the combustion is good, a state where smokeis easily generated, and a state where unburned fuel easily remain,depending upon the operating state of the internal combustion engine.When the exhaust gas heating device of the present invention is used toheat the exhaust gas, the amount of the fuel to be injected into thecombustion chamber of the internal combustion engine from the fuelinjection valve is controlled according, for example, to states such asthe temperature of the exhaust gas, the exhaust flow rate, and theoxygen concentration of the exhaust gas so as to reduce the amount ofthe oxygen to be released into the exhaust passage from the combustionchamber of the internal combustion engine. Accordingly, the operatingstate is set such that it is difficult for smoke and unburned fuel to becontained in the exhaust gas flowing downstream of the exhaust gasheating device in the exhaust passage, thereby allowing the maintenanceof a good combustion state for the fuel to be supplied into the exhaustpassage.

In the exhaust gas heating method according to the present invention,the controlling step may include at least one of the steps of setting anamount of fuel to be injected into the combustion chamber of theinternal combustion engine from a fuel injection valve such that anamount of oxygen to be released into the exhaust passage from thecombustion chamber of the internal combustion engine is reduced, settingan opening degree of a throttle valve, and setting an opening degree ofan EGR control valve. Here, it is effective that the step of setting thefuel supply amount include a step of correcting the fuel supply amountset in the step of setting the fuel supply amount by reducing the fuelsupply amount in accordance with the fuel injection amount set in thestep of setting the fuel injection amount.

The controlling step may include the step of setting the fuel injectionamount, the checking step may include a step of obtaining a temperatureof the exhaust gas, and the exhaust gas heating method may furtherinclude a step of correcting the fuel injection amount set in the stepof setting the fuel injection amount in accordance with the temperatureof the exhaust gas obtained in the step of obtaining the temperature ofthe exhaust gas. Here it is effective that, in the step of correctingthe fuel injection amount, the fuel injection amount set in the step ofsetting the fuel injection amount be corrected by increasing the fuelinjection amount during an operating state in which the temperature ofthe exhaust gas is high, while the fuel injection amount set in the stepof setting the fuel injection amount is corrected by reducing the fuelinjection amount during an operating state in which the temperature ofthe exhaust gas is low.

The controlling step may include the step of setting the fuel injectionamount, the checking step may include a step of obtaining an air-fuelratio of the exhaust gas to be released into the exhaust passage fromthe combustion chamber of the internal combustion engine, and theexhaust gas heating method may further comprise a step of correcting thefuel injection amount set in the step of setting the fuel injectionamount in accordance with the air-fuel ratio of the exhaust gas obtainedin the step of obtaining the air-fuel ratio of the exhaust gas. Here, itis effective that in the step of correcting the fuel injection amount,the fuel injection amount set in the step of setting the fuel injectionamount be corrected by increasing the fuel injection amount when theair-fuel ratio of the exhaust gas is within a lean region, while thefuel injection amount set in the step of setting the fuel injectionamount is corrected by reducing the fuel injection amount when theair-fuel ratio of the exhaust gas is within a rich region.

Preferably, the exhaust gas purifying device includes a DPF (DieselParticulate Filter), and the step of setting the fuel injection amountincludes a step of correcting the fuel injection amount set in the stepof setting the fuel injection amount by reducing the fuel injectionamount when it is judged that fuel needs to be supplied into the exhaustpassage in the determining step during a regeneration process of theDPF.

Advantageous Effects of Invention

The flow rate of the exhaust gas flowing through the exhaust passagelargely varies depending upon the operating state of the internalcombustion engine. Thus, it is extremely difficult to perform accuratecontrol on the combustion of the fuel supplied into the exhaust passagefrom the fuel supply valve without being affected by such variations inthe flow rate of the exhaust gas. However, the present inventioncontrols the air-fuel ratio of the exhaust gas which is released intothe exhaust passage from the combustion chamber of the internalcombustion engine when the fuel is supplied into the exhaust passage.Thus, the air-fuel ratio can fall within an area where the amounts ofsmoke and HC to be generated are both low. For example, when the exhaustgas heating device heats the exhaust gas in a state where the fuelinjection into the combustion chamber of the internal combustion engineis stopped, such as during deceleration of the vehicle, fuel is suppliedinto the combustion chamber of the internal combustion engine withoutcombustion and is mixed with the exhaust gas. This lowers the oxygenconcentration of the exhaust gas, thereby making it possible to suppressthe generation of smoke.

When the controlling step includes at least one of the steps of settingthe fuel injection amount, setting the throttle valve opening degree,and setting the EGR control valve opening degree, the air-fuel ratio ofthe exhaust gas to be released into the exhaust passage from thecombustion chamber of the internal combustion engine can be controlledrapidly and accurately.

When the fuel supply amount set in the step of setting the fuel supplyamount is corrected by reducing it in accordance with the fuel injectionamount set in the step of setting the fuel injection amount, the releaseof unburned fuel can further be suppressed.

Similarly, the release of unburned fuel can further be suppressed aswell when the fuel injection amount set in the step of setting the fuelinjection amount is corrected according to the exhaust gas temperatureobtained in the step of obtaining the exhaust gas temperature.Specifically, the fuel injection amount is corrected by increasing itduring an operating state in which the exhaust gas temperature is high,while the fuel injection amount set in the step of setting the fuelinjection amount is corrected by reducing it during an operating statein which the exhaust gas temperature is low. In this way, the generationof smoke can be suppressed together with the suppression of unburnedfuel.

When the exhaust gas heating method further includes the step ofcorrecting the fuel injection amount set in the step of setting the fuelinjection amount in accordance with the air-fuel ratio of the exhaustgas obtained in the step of obtaining the air-fuel ratio of the exhaustgas, an increase in the amount of unburned fuel can further besuppressed. Specifically, the fuel injection amount is corrected byincreasing it in a case where the air-fuel ratio of the exhaust gas iswithin a lean region, while the fuel injection amount is corrected byreducing it in a case where the air-fuel ratio of the exhaust gas iswithin a rich region. In this way, the generation of smoke can besuppressed together with the suppression of unburned fuel.

When the exhaust gas purifying device includes a DPF, and the set fuelsupply amount is corrected by reducing it in a case where fuel issupplied into the exhaust passage during a regeneration process of theDPF, the fuel consumption can be suppressed. Moreover, since the DPF isin the regeneration process, smoke generated can be removed throughoxidation thereof. Furthermore, the burnability of PM (ParticulateMatter) can be enhanced along with an increase in the oxygenconcentration of the exhaust gas flowing into the DPF.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a conceptual diagram of an embodiment of an engine systemwhich is capable of implementing an exhaust gas heating method of thepresent invention;

FIG. 2 is a control block diagram of a main part of the embodiment shownin FIG. 1;

FIG. 3 is a graph schematically showing the relationship between theamount of fuel injected into a cylinder and the amounts of smoke and HCgenerated;

FIG. 4 is a graph schematically showing the relationship between theexhaust gas temperature and an area to be selected for a target air-fuelratio;

FIG. 5 is a graph schematically showing the relationships between theexhaust gas temperature and oxygen concentration, and an area involvingthe generation of a large amount of smoke, an area involving thegeneration of a large amount of HC, and a preferable combustion area;

FIG. 6 is a graph schematically showing the relationships between theexhaust gas temperature and air-fuel ratio, and an area involving thegeneration of a large amount of smoke and a preferable combustion area;

FIG. 7 is graph schematically showing the relationships between theintake flow rate and air-fuel ratio, and an area involving thegeneration of a large amount of smoke and a preferable combustion area;

FIG. 8 is a flowchart showing a procedure related to fuel additioncontrol of the embodiment;

FIG. 9 is a flowchart showing a specific procedure in afuel-injection-amount setting step in FIG. 8;

FIG. 10 is a flowchart showing a specific procedure in afuel-addition-amount setting step in FIG. 8; and

FIG. 11 is a flowchart showing a specific procedure in aninjection-amount air-fuel ratio correcting step in FIG. 8.

DESCRIPTION OF EMBODIMENTS

An embodiment of a compression-ignition internal combustion enginecapable of implementing an exhaust gas heating method of the presentinvention will be described in detail with reference to FIGS. 1 to 11.Note that the present invention is not limited to this embodiment, andits configuration can be changed freely according to the specifications,characteristics, and the like required.

FIG. 1 schematically shows a main part of an engine system of thisembodiment, and FIG. 2 shows a control block thereof. It is to be notedthat a valve gear and the like for air intake and exhaust to and from anengine 10 are omitted in FIG. 1 for simplicity and convenience.

The engine 10 of this embodiment is an internal combustion engine of acompression ignition type which causes spontaneous ignition of dieseloil, serving as fuel, by injecting the diesel oil from a fuel injectionvalve 11 directly into a combustion chamber 10 a being in a compressedstate.

An ECU (Electronic Control Unit) 13 controls the amount of fuel to besupplied into the combustion chamber 10 a from the fuel injection valve11 (hereinafter, referred to as the fuel injection amount) and thetiming of the injection on the basis of the vehicle's operating stateincluding the amount of the driver's depression of an accelerator pedal12. An accelerator opening sensor 14 detects this amount of depressionof the accelerator pedal 12 (hereinafter, referred to as the acceleratoropening degree) GA and outputs the detected information to the ECU 13.

In this embodiment, two injection modes are set for the fuel injectioninto the combustion chamber 10 a from the fuel injection valve 11.Specifically, one is an engine driving mode in which fuel is subjectedto compression ignition and combusted inside the combustion chamber 10 ato obtain an output of the engine 10. The other is an exhaust gasheating mode in which injected fuel is not ignited inside the combustionchamber 10 a but is flowed as it is into a later-described exhaustpassage 23 a. FIG. 3 schematically shows the relationship between thefuel injection amount and the amounts of smoke and unburned HC in theexhaust gas in the exhaust gas heating mode. As is obvious from FIG. 3,there is a tendency that the generated smoke increases as the fuelinjection amount decreases, and conversely the amount of unburned HCincreases as the fuel injection amount increases. Thus, the smoke and HCcan both be suppressed to a small amount by setting the fuel injectionamount within an appropriate smoke/HC reduction range given in FIG. 3.

In this embodiment, when a fuel injection process is needed during theexhaust gas heating mode, the fuel injection amount is set to F_(M)which is situated substantially at the center of the smoke/HC reductionrange. However, when a PM burning rate V_(B) is higher than a rate V_(D)of PM deposition onto a DPF 26 b of a later-described exhaust gaspurifying device 26 during a regeneration process of the DPF 26 b, smokewill be processed in the DPF 26 b even if a large amount of smoke isgenerated. Thus, in this case, the fuel injection amount is corrected byreducing it by ΔF_(M) so as to suppress unnecessary fuel consumption andalso suppress the generation of unburned HC.

Moreover, the amounts of smoke and unburned HC to be generated vary whenan exhaust gas temperature T_(E) is too low or too high. Thus, the fuelinjection amount is further corrected according to the exhaust gastemperature T_(E). More specifically, when the exhaust gas temperatureT_(E) is below a preset first threshold temperature T_(L), the amount ofsmoke to be generated is highly likely to decrease, and therefore thefuel injection amount is corrected by reducing it by ΔF_(T), therebysuppressing an increase in the amount of unburned HC to be generated. Onthe other hand, when the exhaust gas temperature T_(E) is higher than apreset second threshold temperature T_(M), the amount of smoke to begenerated is highly likely to increase, and therefore the fuel injectionamount is corrected by increasing it by ΔF_(T), thereby suppressing anincrease in the amount of smoke to be generated.

Likewise, in this embodiment, the amounts of smoke and unburned HC to begenerated vary also when an actual air-fuel ratio R_(N) differs greatlyfrom a target air-fuel ratio R_(T) by, for example, ±ΔR. Thus, the fuelinjection amount is further corrected according to the air-fuel ratioR_(N). More specifically, when the air-fuel ratio R_(N) is leaner by ΔRthan the target air-fuel ratio R_(T), smoke is easily generated, andtherefore the fuel injection amount is corrected by increasing it byΔF_(R). On the other hand, when the air-fuel ratio R_(N) is richer by ΔRthan the target air-fuel ratio R_(T), unburned HC is easily generated,and therefore the fuel injection amount is corrected by reducing it byΔF_(R).

Note that instead of the fuel-injection-amount correcting processesdescribed above or in addition to the fuel-injection-amount correctingprocesses, at least one of the air intake amount and the EGR amount maybe controlled. In this way too, a similar advantageous effect can beachieved.

The ECU 13 includes an operating status determining section 13 a, a fuelinjection setting section 13 b, a fuel injection valve driving section13 c, a PM deposition rate calculating section 13 d, and a PM burningrate calculating section 13 e. The operating status determining section13 a checks the operating state of the vehicle on the basis ofinformation from the accelerator opening sensor 14, later-describedvarious sensors, and the like. The fuel injection setting section 13 bsets the amount of fuel to be injected from the fuel injection valve 11and the timing of the injection on the basis of the result of thedetermination by the operating status determining section 13 a. Notethat the fuel injection setting section 13 b basically sets the fuelinjection amount to F_(M) when the exhaust gas heating mode is selected.The fuel injection valve driving section 13 c controls the actuation ofthe fuel injection valve 11 such that fuel is injected from the fuelinjection valve 11 by the amount and at the timing set by the fuelinjection setting section 13 b. The PM deposition rate calculatingsection 13 d calculates the PM deposition rate V_(D) during the DPFregeneration process, and the PM burning rate calculating section 13 ecalculates the PM burning rate V_(B) during the DPF regenerationprocess. These calculations are performed by use of well-known methods.

In a cylinder head 15 forming therein an intake port 15 a and an exhaustport 15 b each in communication with the combustion chamber 10 a, theunillustrated valve gear is installed which includes an intake valve 16a and an exhaust valve 16 b which open and close the intake port 15 aand the exhaust port 15 b, respectively. The aforementioned fuelinjection valve 11 is installed in this cylinder head 15 as well.

In an intake pipe 17 joined to the cylinder head 15 in communicationwith the intake port 15 a and defining an intake passage 17 a togetherwith the intake port 15 a, a throttle valve 19 is installed with whichto adjust the opening degree of the intake passage 17 a with a throttleactuator 18.

The aforementioned ECU 13 further includes a throttle opening settingsection 13 f and an actuator driving section 13 g. The throttle openingsetting section 13 f sets the opening degree of the throttle valve 19 onthe basis of the result of the checking by the aforementioned operatingstatus determining section 13 a. The actuator driving section 13 gcontrols the actuation of the throttle actuator 18 such that thethrottle valve 19 is set at the opening degree set by the throttleopening setting section 13 f.

In a cylinder block 21 in which a piston 20 a reciprocates, a crankangle sensor 22 is mounted which detects the rotational phase of acrankshaft 20 c, i.e. the crank angle and outputs it to the ECU 13. Thepiston 20 a is joined to the crankshaft 20 c through a connecting rod 20b.

Based on the information from this crank angle sensor 22, the operatingstatus determining section 13 a of the ECU 13 checks the rotationalphase of the crankshaft 20 c and the engine speed as well as the travelspeed of the vehicle and the like on a real-time basis.

Installed in the engine 10 are an EGR system 24, a exhaust turbocharger25, the exhaust gas purifying device 26, and an exhaust gas heatingdevice 27. The EGR system 24 introduces part of the exhaust gas, flowinginside the exhaust passage 23 a, back into the intake passage 17 a.

The EGR system 24 for primarily reducing nitrogen oxides in the exhaustgas includes an EGR pipe 28 which defines an EGR passage 28 a and an EGRcontrol valve 29 which is provided to this EGR pipe 28 to control theflow rate of the exhaust gas flowing through the EGR passage 28 a. TheEGR pipe 28 is in communication at one end with an exhaust pipe 23defining the exhaust passage 23 a together with the exhaust port 15 b,and is in communication at the other end with a portion of the intakepassage 17 a between the intake port 15 a and a surge tank 17 b disposeddownstream of the above-mentioned throttle valve 19.

In this embodiment, when the operating status determining section 13 aof the ECU 13 determines that the vehicle equipped with the engine 10 iswithin a preset EGR operation region, the EGR amount setting section 13h of the ECU 13 sets the opening degree of the EGR control valve 29 inaccordance with the operating state of the vehicle of that moment. TheEGR valve driving section 13 i of the ECU 13 controls the EGR controlvalve 29 at the opening degree set by the EGR amount setting section 13h. Besides the above case, the EGR valve driving section 13 i basicallydrives the EGR control valve 29 such that the EGR control valve 29shifts to a closed state to close the EGR passage 28 a.

The exhaust turbocharger (hereinafter, simply referred to as thesupercharger) 25 utilizes the kinetic energy of the exhaust gas flowingthrough the exhaust passage 23 a to supercharge the combustion chamber10 a, thereby enhancing the air charging efficiency. The main part ofthe supercharger 25 of this embodiment is formed of an intake turbine 25a and an exhaust turbine 25 b which rotates together with this intaketurbine 25 a. The intake turbine 25 a is installed in an intermediateportion of the intake pipe 17 which is located upstream of the throttlevalve 19. The exhaust turbine 25 b is installed in an intermediateportion of the exhaust pipe 23 joined to the cylinder head 15 incommunication with the exhaust port 15 b. In this embodiment, anintercooler 25 c is installed in an intermediate portion of the intakepassage 17 a between the intake turbine 25 a and the surge tank 17 b soas to lower the temperature of the intake air heated by heat transferfrom the exhaust turbine 25 b side through the intake turbine 25 a.

An air flow meter 30 is provided upstream of the intake turbine 25 a ofthe supercharger 25 in the intake pipe 17 and detects the flow rate ofthe intake air flowing through the intake passage 17 a of this portionand outputs it to the ECU 13. Note that the one end of the EGR pipe 28mentioned above is connected to a portion of the exhaust pipe 23upstream of the exhaust turbine 25 b.

The exhaust gas purifying device 26 for detoxifying toxic substancesgenerated by the combustion of an air-fuel mixture in the combustionchamber 10 a is installed in a portion of the exhaust pipe 23 defining aportion of the exhaust passage 23 a downstream of the exhaust turbine 25b of the supercharger 25. The exhaust gas purifying device 26 of thisembodiment includes an oxidation catalytic converter 26 a and the DPF(Diesel Particulate Filter) 26 b both being well known. Note that NOX(Nitrogen Oxides) catalytic converter or the like may further beincluded.

In this exhaust gas purifying device 26, a catalyst temperature sensor31 is installed which detects a temperature T_(C) thereof (hereinafter,referred to as the catalyst temperature) and outputs it to the ECU 13.Moreover, an exhaust gas temperature sensor 32 is mounted upstream ofthe exhaust gas purifying device 26 and downstream of the exhaust gasheating device 27 in the exhaust pipe 23. This exhaust gas temperaturesensor 32 detects a temperature T_(E) of the exhaust gas flowing througha portion of the exhaust passage 23 a immediately before the exhaust gaspurifying device 26 and outputs the detected information to the ECU 13.

Based on the information from the catalyst temperature sensor 31 and theexhaust gas temperature sensor 32, the ECU 13 of this embodimentdetermines the necessity for actuating the exhaust gas heating device27, i.e. whether or not fuel needs to be supplied. The ECU 13 determinesthat fuel needs to be supplied normally when the catalyst temperatureT_(C) is or is expected to be lower than a reference being the lowestpossible temperature for the oxidation catalyst 26 a to maintain itsactive state. However, some other conventionally well-known determiningmethod can be employed optionally.

The exhaust gas heating device 27, the actuation of which is controlledbased on the operating state of the vehicle and the state of the exhaustgas purifying device 26, is configured to heat the exhaust gas to beintroduced into the exhaust gas purifying device 26 from the engine 10to thereby rapidly activate the exhaust gas purifying device 26 andmaintain its active state. The exhaust gas heating device 27 of thisembodiment includes a fuel supply valve 27 a and a glow plug 27 b.

The fuel supply valve 27 a for supplying fuel to activate the exhaustgas purifying device 26 or to maintain its active state is mounted tothe exhaust pipe 23 in such a way as to face a portion of the exhaustpassage 23 a located downstream of the exhaust turbine 25 b of thesupercharger 25 but upstream of the exhaust gas purifying device 26.When the exhaust gas purifying device 26 comes in need of heating, oractivation, fuel is supplied from this fuel supply valve 27 a into theexhaust passage 23 a by actuating the exhaust gas heating device 27basically in a low-load operating state. The glow plug 27 b for ignitingthe fuel supplied from the fuel supply valve 27 a into the exhaustpassage 23 a has a heating portion on its tip side disposed downstreamof the fuel supply valve 27 a in the exhaust passage 23 a. The fuel fromthe fuel supply valve 27 a is supplied toward this heating portion. Theglow plug 27 b is connected to an unillustrated on-vehicle power supplythrough an unillustrated switch whose ON and OFF are controlled by theECU 13. A glow plug driving section 13 m of the ECU 13 switches the ONand OFF of the actuation of the glow plug 27 b on the basis ofinformation on the drive of the fuel supply valve 27 a from a fuelsupply valve driving section 13 l.

The ECU 13 further includes a target air-fuel ratio setting section 13j, a fuel supply setting section 13 k, and the fuel supply valve drivingsection 13 l.

The target air-fuel ratio setting section 13 j sets the target air-fuelratio R_(T) of the exhaust gas which flows into the exhaust gaspurifying device 26 when the exhaust gas is to be heated in an exhaustgas heating process. In this embodiment, an air-fuel ratio allowingefficient fuel combustion in the exhaust gas heating process, i.e. thetarget air-fuel ratio RT is set based on the exhaust gas temperatureT_(E) detected by the exhaust gas temperature sensor 32 and the oxygenconcentration of the exhaust gas flowing into the exhaust gas heatingdevice 27 from the combustion chamber 10 a. FIG. 4 schematically showsthe relationship between the exhaust gas temperature T_(E) and theair-fuel ratio. FIGS. 5 to 7 schematically show the relationshipsbetween the intake flow rate, air-fuel ratio, oxygen concentration andexhaust gas temperature T_(E) and regions allowing efficient fuelcombustion in the exhaust gas heating process (hereinafter, referred toas the preferable combustion regions for convenience). Note that whilethe oxygen concentration is calculated by the operating statusdetermining section 13 a of the ECU, a well-known O₂ sensor may beinstalled upstream of the exhaust gas heating device 27 in the exhaustpassage 23 a. The preferable combustion region is a region enclosed by adashed line in each of FIGS. 5 to 7. By the target air-fuel ratiosetting section 13 j, a single value of the target air-fuel ratio R_(T)is set to such a value as to fall within each of these regions enclosedby the dashed line.

The fuel supply setting section 13 k sets the amount of fuel to besupplied into the exhaust passage 23 a from the fuel supply valve 27 a.More specifically, based on the difference between a target catalysttemperature T_(T) and the catalyst temperature T_(C) detected by thecatalyst temperature sensor 31, the fuel supply setting section 13 kcalculates the amount of fuel to be supplied into the exhaust passage 23a in the exhaust gas heating process for heating the exhaust gas, i.e. atarget fuel supply amount F_(T). In general, selected as the targetcatalyst temperature T_(T) is the lowest possible temperature for theoxidation catalyst 26 a to be in an active state. The fuel supplysetting section 13 k also sets an amount F_(U) of fuel to be suppliedinto the exhaust passage 23 a in a single current application to thefuel supply valve 27 a (hereinafter, referred to as the unit supplyamount), as well as the cycle of the current application (hereinafter,referred to as the supply cycle) t_(P). The unit supply amount F_(U) isset based on an exhaust flow rate which is based on the information fromthe air flowmeter 30. Basically, the larger the exhaust flow rate is,the larger the unit supply amount F_(U) is set. The supply cycle t_(P)is set such that adding the set unit supply amount F_(U) of fuel intothe exhaust passage 23 a results in the aforementioned target air-fuelratio R_(T).

Note that in a case where the fuel addition from the fuel supply valve27 a and the fuel injection from the fuel injection valve 11 are to beperformed simultaneously, at least one of the target fuel supply amountF_(T) and the unit supply amount F_(U) is corrected by reducing it inaccordance with the amount of the fuel to be injected from the fuelinjection valve 27 a. Moreover, in a case where the actual air-fuelratio R_(N) differs from the target air-fuel ratio R_(T), air-fuel ratiocontrol is performed to make the actual air-fuel ratio R_(N) match thetarget air-fuel ratio R_(T), and at least one the fuel injection amount,the air intake amount, and the EGR amount is corrected if necessary.

The fuel supply valve driving section 13 l controls the actuation of thefuel supply valve 27 a such that fuel is supplied into the exhaustpassage 23 a by the unit supply amount at the supply cycle set by thefuel supply setting section 13 k.

In this embodiment, the operating status determining section 13 acalculates the air-fuel ratio R_(N) of the exhaust gas immediatelybefore flowing into the exhaust gas purifying device 26 on the basis ofthe air intake amount detected by the air flow meter 30, the fuelinjection amount of the fuel injection valve 11, and the fuel supplyamount of the fuel supply valve 27 a. Alternatively, an air-fuel ratiosensor can be disposed downstream of the exhaust gas heating device 27and upstream of the exhaust gas purifying device 26 in the exhaustpassage 23 a, and the air-fuel ratio R_(N) can be detected by use of adetection signal from the air-fuel ratio sensor.

As described, by introducing exhaust gas containing unburned fuel intothe exhaust gas heating device 27, it is possible to minimize theconcentration of the oxygen contained in the exhaust gas sent out to theexhaust gas heating device 27 from the combustion chamber 10 a of theengine 10, and thereby to reduce the amount of smoke to be generated.

The ECU 13 of this embodiment is a well-known one-chip microprocessorand includes a CPU, a ROM, a RAM, a nonvolatile memory, an I/Ointerface, and the like which are connected to each other by anunillustrated data bus. This ECU 13 performs predetermined arithmeticprocessing on the basis of the detection signals from theabove-mentioned sensors 14, 22, 31, and 32, air flow meter 30, and thelike so that the engine 10 can operate smoothly. Moreover, the ECU 13controls the actuations of the fuel injection valve 11, the throttlevalve 19, the EGR control valve 29, the fuel supply valve 27 a, the glowplug 27 b, and the like in accordance with a preset program.

Accordingly, the intake air supplied into the combustion chamber 10 athrough the intake passage 17 a form an air-fuel mixture together withthe fuel injected into the combustion chamber 10 a from the fuelinjection valve 11. Then, normally, the air-fuel mixture undergoesspontaneous ignition and combustion immediately before the piston 20 areaches top dead center of the compression stroke. The resultant exhaustgas generated passes through the exhaust gas purifying device 26 and isreleased in a detoxified state into the atmosphere through the exhaustpipe 23.

Meanwhile, the exhaust gas heating process of the present invention isperformed while the engine 10 is in operation, in accordance with thestate of the exhaust gas purifying device 26. Using the flowcharts inFIGS. 8 to 11, a procedure for actuating the exhaust gas heating device27 in this embodiment will be described. First, in step S11, it isdetermined whether or not fuel needs to be supplied. If it is judged inthis step that fuel needs to be supplied, that is, the exhaust gaspurifying device 26 needs to be activated, the procedure proceeds tostep S12, where it is judged whether or not the current operating stateis within an area involving the generation of a large amount of smoke.If it is judged in this step that the current operating state is withinthe generation area of a large amount of smoke, that is, it is desirableto inject fuel into the combustion chamber 10 a from the fuel injectionvalve 11, the procedure proceeds to step S13, where a fuel injectionamount setting process is performed.

Specifically, in step S131 in FIG. 9, the fuel injection amount is setto F. Then, in step S132, it is determined whether or not the DPF 26 bis in a regeneration process. If it is judged in this step that the DPF26 b is in a regeneration process, the procedure proceeds to step S133,where it is determined whether or not the PM burning rate V_(B) is equalto or greater than the PM deposition rate V_(D). If it is judged in thisstep that the PM burning rate V_(B) is equal to or greater than the PMdeposition rate V_(D), that is, PM is decreasing due to the regenerationprocess of the DPF 26 b, the procedure proceeds to step S134. There, thefuel injection amount F_(M) set in the step S131 is corrected byreducing it by ΔF_(M), thus suppressing an increase in the amount ofunburned HC to be generated.

Thereafter, in step S135, it is determined whether or not the exhaustgas temperature T_(E) is equal to or higher than the first thresholdtemperature T_(L) but is equal to or lower than the second thresholdtemperature T_(H). If it is judged in this step that the exhaust gastemperature T_(E) is equal to or higher than the first thresholdtemperature T_(L) but is equal to or lower than the second thresholdtemperature T_(H), that is, the exhaust gas temperature T_(E) is not toolower or too high, this subroutine for setting the fuel injection amountis terminated, and the procedure proceeds to step S14 in the mainflowchart shown in FIG. 8. On the other hand, if it is judged in thestep S135 that the exhaust gas temperature T_(E) is not equal to orhigher than the first threshold temperature T_(L) or not equal to orlower than the second threshold temperature T_(M), the procedureproceeds to step S136. This time, it is determined whether or not theexhaust gas temperature T_(E) is higher than the second thresholdtemperature T_(H). If it is judged in this step that the exhaust gastemperature T_(E) is higher than the second threshold temperature T_(H),that is, the amount of smoke to be generated will be greater thaninitially expected, then, in step S137, the fuel injection amount F_(M)set in the step S131 is corrected by increasing it by ΔF_(T). On theother hand, if it is judged in the step S136 that the exhaust gastemperature T_(E) is not higher than the second threshold temperatureT_(H), that is, the exhaust gas temperature T_(E) is lower than thefirst threshold temperature T_(L) and therefore the amount of smoke tobe generated will be less than initially expected, the procedureproceeds to step S138. There, the fuel injection amount F_(M) set in thestep S131 is corrected by reducing it by ΔF_(T), thus suppressing anincrease in the amount of unburned HC to be generated.

When the exhaust gas temperature T_(E) is not equal to or higher thanfirst threshold temperature T₁, or not equal to or lower than secondthreshold temperature T_(H), the fuel injection amount F_(M) set in thestep S131 is corrected in the step S137 or S138 as described above, andthen the fuel injection amount setting process in FIG. 9 is terminated.Thereafter, the procedure proceeds to the step S14 in FIG. 8.

In the step S14, it is determined whether or not the accelerator openingdegree θ_(A) is 0. If it is judged in this step that the acceleratoropening degree θ_(A) is 0, that is, the vehicle is decelerating oridling and no fuel is being injected into the combustion chamber 10 afrom the fuel injection valve 11, the procedure proceeds to step S15. Inthis step, injection of fuel from the fuel injection valve 11 into thecombustion chamber 10 a is started. The fuel is released to the exhaustpassage 23 a without being combusted inside the combustion chamber 10 a,thereby lowering the oxygen concentration of the exhaust gas. Asdescribed, in this embodiment, fuel is injected into the combustionchamber 10 a from the fuel injection valve 11 only when the vehicle isdecelerating or idling. This makes it possible to minimize an awkwardfeel the driver experiences due to torque fluctuations of the engine 10.After step S15, it is determined in step S16 whether or not a flag isset. At first, a flag is not set, and the procedure therefore proceedsto step S17, where a timer is caused to count up, and it is determinedin step S18 whether or not a count value C_(N) of the timer is equal toor greater than a target count value C_(T). Subsequently, the procedurereturns to the step S11 from the step S18 and the above-mentionedprocesses are basically repeated until the count value C_(N) of thetimer reaches the target count value C_(T).

When it is eventually judged in the step S18 that the count value C_(N)of the timer is equal to or great than the target count value C_(T),that is, the exhaust gas containing the fuel injected from the fuelinjection valve 11 has reached the exhaust gas heating device 27, theprocedure proceeds to step S19, where a flag is set. Then, the procedureproceeds to step S20, where a fuel addition amount setting process isperformed.

Meanwhile, in the step S12 mentioned earlier, if it is judged that thecurrent operating state is not within the generation area of a largeamount of smoke, that is, it is not necessary to inject fuel into thecombustion chamber 10 a, the procedure proceeds to step S21, where it isdetermined whether or not the current operating state is within anoperating area involving the generation of a large amount of unburnedHC. If it is judged in this step that the current operating state iswithin the generation area of a large amount of unburned HC, theprocedure proceeds to the step S134 in FIG. 9, where the fuel injectionamount F_(M) set in the step S131 is corrected by reducing it by ΔF_(M),thus suppressing the generation of unburned HC. Moreover, if it isjudged in the step S21 that the current operating state is not withinthe generation area of a large amount of unburned HC, that is, it is notnecessary to inject fuel into the combustion chamber 10 a from the fuelinjection valve 11, the procedure proceeds directly to the step S20.Note that the procedure proceeds to the step S20 also when it isdetermined in the step S16 that a flag is set, that is, the exhaust gascontaining the fuel injected from the fuel injection valve 11 hasreached the exhaust gas heating device 27.

In a subroutine for setting the fuel supply amount in the step S20 shownin FIG. 10, first, in step S201, the target fuel supply amount F_(T) isset according to the difference between the catalyst temperature T_(C)and the target catalyst temperature T_(T). Then, in step S202, thetarget air-fuel ratio R_(T) is set. Thereafter, in step S203, the unitsupply amount F_(U) is calculated. Subsequently, in step S204, thesupply cycle t_(P) is set according to the unit supply amount F_(U) andthe target air-fuel ratio R_(T), and then the fuel-addition-amountsetting process is terminated. The procedure then proceeds to step S22in the main flowchart shown in FIG. 8.

In this step S22, fuel is supplied into the exhaust passage 23 a fromthe fuel supply valve 27 a. The fuel is ignited and combusted by use ofthe ignition means, and the resultant hot exhaust gas is introduced intothe exhaust gas purifying device 26. In this case, fuel is also injectedinto the combustion chamber 10 a from the fuel injection valve 11 asneeded so as to suppress the generation of both smoke and HC to a smallamount. Accordingly, harmful effects of the smoke and HC on the exhaustgas purifying device 26 can be minimized. Thereafter, in step S23, it isdetermined whether or not a flag is set. If it is judged in this stepthat a flag is set, that is, fuel is also injected into the combustionchamber 10 a from the fuel injection valve 11, the procedure proceeds tostep S24, where an injection-amount air-fuel ratio correcting process isperformed.

In a subroutine for correcting the injection amount and the air-fuelratio shown in FIG. 11, first, in step S241, it is determined whether ornot the current air-fuel ratio R_(N) is within a predetermined regionwith respect to the target air-fuel ratio R_(T). If it is judged in thisstep that the current air-fuel ratio R_(N) is within the predeterminedregion with respect to the target air-fuel ratio R_(T), that is, it isnot necessary to correct the current air-fuel ratio, the injectionamount air fuel ratio correcting process in FIG. 11 is terminated. Theprocedure then proceeds to step S25 in FIG. 8. On the other hand, if itis judged in the step S241 that the current air-fuel ratio R_(N) isoutside the predetermined region with respect to the target air-fuelratio R_(T), the procedure proceeds to step S242, where it is determinedwhether or not the current air-fuel ratio R_(N) is greater than thetarget air-fuel ratio R_(T) by ΔR or greater. If it is judged in thisstep that the current air-fuel ratio R_(N) is greater than the targetair-fuel ratio R_(T) by ΔR or greater, that is, the air-fuel ratio is solean that smoke is easily generated, then in step S243, the fuelinjection amount set in the step S13 is corrected by increasing it byΔF_(R). If it is judged in the step S242 that the current air-fuel ratioR_(N) is not greater than the target air-fuel ratio R_(T) by ΔR, thatis, the air-fuel ratio is so rich that HC is easily generated, then instep S244, the fuel injection amount F_(M) set in the step S13 iscorrected by reducing it by ΔF_(R).

The procedure proceeds to step S25 once the current air-fuel ratio R_(N)is controlled to fall within the predetermined region with respect tothe target air-fuel ratio R_(T) by correcting the fuel injection amountof the fuel injection valve 11 in accordance with the difference betweenthe current air-fuel ratio R_(N) and the target air-fuel ratio R_(T) asdescribed above.

Meanwhile, in the step S23 mentioned earlier, if it is judged that the aflag is not set, that is, fuel is not injected from the fuel injectionvalve 11, the procedure also proceeds to the step S25, where it isdetermined whether or not the catalyst temperature T_(C) has becomeequal to or higher than the target catalyst temperature T_(T). If it isjudged in this step that the catalyst temperature T_(C) is lower thanthe target catalyst temperature T_(T), that is, it is necessary tocontinue heating the exhaust gas, the procedure returns to the step S11and the above mentioned processes are repeated. On the other hand, if itis judged that the catalyst temperature T_(C) is equal to or higher thanthe target catalyst temperature T_(T), that is, the exhaust gaspurifying device 26 has shifted to an active state and the heating is nolonger necessary, the procedure proceeds to step S26. Then, the exhaustgas heating process is terminated, and the count value C_(N) of thetimer is reset to 0, and thereafter the procedure returns to the initialstep S11. Moreover, the procedure proceeds to the step S26 also when theaccelerator opening degree θ_(A) is not 0 in the step S14, that is, thedriver is depressing the accelerator pedal 12 in the step S14. This isbecause the driver's depression of the accelerator pedal 12 starts thecombustion of fuel inside the combustion chamber 10 a and thus raisesthe exhaust gas temperature T_(E), thereby eliminating the need forperforming the exhaust gas heating process. Similarly, when it is judgedin the step S11 mentioned earlier that fuel does not need to beinjected, the procedure also proceeds to step S26, where the exhaust gasheating process is terminated and the count value C_(N) of the timer isreset to 0.

It is to be noted that the present invention shall be construed solelyfrom the matters described in the claims thereof, and the foregoingembodiment includes not only the matters described above but any changesand corrections encompassed by the concept of the present invention. Inother words, all the matters in the foregoing embodiment are not tolimit the present invention, but include any configurations which maynot be directly related to the present invention and can be changedoptionally depending upon the application, purpose, and the like.

REFERENCE SIGNS LIST

-   10 engine-   10 a combustion chamber-   11 fuel injection valve-   12 accelerator pedal-   13 ECU-   13 a operating status determining section-   13 b fuel injection setting section-   13 c fuel injection valve driving section-   13 d PM deposition rate calculating section-   13 e PM burning rate calculating section-   13 f throttle opening setting section-   13 g actuator driving section-   13 h EGR amount setting section-   13 i EGR valve driving section-   13 j target air-fuel ratio setting section-   13 k fuel supply setting section-   13 l fuel supply valve driving section-   13 m glow plug driving section-   14 accelerator opening sensor-   15 cylinder head-   15 a intake port-   15 b exhaust port-   16 a intake valve-   16 b exhaust valve-   17 intake pipe-   17 a intake passage-   17 b surge tank-   18 throttle actuator-   19 throttle valve-   20 a piston-   20 b connecting rod-   20 c crankshaft-   21 cylinder block-   33 crank angle sensor-   23 exhaust pipe-   23 a exhaust passage-   24 EGR system-   25 exhaust turbocharger-   25 a intake turbine-   25 b exhaust turbine-   25 c intercooler-   26 exhaust gas purifying device-   26 a oxidation catalytic converter-   26 b DPF-   27 exhaust gas heating device-   27 a fuel supply valve-   27 b glow plug-   28 EGR pipe-   28 a EGR passage-   29 EGR control valve-   30 air flow meter-   31 catalyst temperature sensor-   32 exhaust gas temperature sensor-   θ_(A) accelerator opening degree-   C_(N) count value of timer-   C_(T) target count value-   F_(M) fuel injection amount-   F_(T) target fuel supply amount-   F_(U) unit supply amount-   ΔF_(M) fuel-injection correcting amount-   T_(C) catalyst temperature-   T_(E) exhaust gas temperature-   T_(H) second threshold temperature-   T_(L) first threshold temperature-   T_(T) target catalyst temperature-   R_(N) actual air-fuel ratio-   R_(T) target air-fuel ratio-   V_(B) PM burning rate-   V_(D) PM deposition rate-   t_(P) supply cycle

1.-8. (canceled)
 9. A method of heating exhaust gas to be introducedinto an exhaust gas purifying device from an internal combustion engineby supplying fuel into an exhaust passage from a fuel supply valvedisposed upstream of the exhaust gas purifying device in an exhaust pipeupstream, and then by heating and igniting the fuel supplied into theexhaust passage, the method comprising steps of: determining a necessityfor supplying fuel into the exhaust passage; setting an amount of fuelto be supplied into the exhaust passage from the fuel supply valve, theamount being set when it is judged in the determining step that fuelneeds to be supplied into the exhaust passage; checking an operatingstate of the internal combustion engine; and controlling an air-fuelratio of exhaust gas to be released into the exhaust passage from acombustion chamber of the internal combustion engine such that theair-fuel ratio falls within an area where amounts of both smoke and HCto be generated in the exhaust gas flowing through the exhaust passageare low in the operating state of the internal combustion engine checkedin the checking step.
 10. The method as claimed in claim 9, wherein thecontrolling step includes at least one of steps of: setting an amount offuel to be injected into the combustion chamber of the internalcombustion engine from a fuel injection valve such that an amount ofoxygen to be released into the exhaust passage from the combustionchamber of the internal combustion engine is reduced; setting an openingdegree of a throttle valve; and setting an opening degree of an EGRcontrol valve.
 11. The method as claimed in claim 10, wherein the stepof setting the fuel supply amount includes a step of correcting the fuelsupply amount set in the step of setting the fuel supply amount byreducing the fuel supply amount in accordance with the fuel injectionamount set in the step of setting the fuel injection amount.
 12. Themethod as claimed in claim 10, wherein the controlling step includes thestep of setting the fuel injection amount, the checking step includes astep of obtaining a temperature of the exhaust gas, and the methodfurther comprising a step of correcting the fuel injection amount set inthe step of setting the fuel injection amount in accordance with thetemperature of the exhaust gas obtained in the step of obtaining thetemperature of the exhaust gas.
 13. The method as claimed in claim 12,wherein in the step of correcting the fuel injection amount, the fuelinjection amount set in the step of the fuel injection amount iscorrected by increasing the fuel injection amount during an operatingstate in which the temperature of the exhaust gas is high, while thefuel injection amount set in the step of setting the fuel injectionamount is corrected by reducing the fuel injection amount during anoperating state in which the temperature of the exhaust gas is low. 14.The method as claimed in claim 10, wherein the controlling step includesthe step of setting the fuel injection amount, the checking stepincludes a step of obtaining an air-fuel ratio of the exhaust gas to bereleased into the exhaust passage from the combustion chamber of theinternal combustion engine, and the method further comprising a step ofcorrecting the fuel injection amount set in the step of setting the fuelinjection amount in accordance with the air-fuel ratio of the exhaustgas obtained in the step of obtaining the air-fuel ratio of the exhaustgas.
 15. The method as claimed in claim 14, wherein in the step ofcorrecting the fuel injection amount, the fuel injection amount set inthe step of setting the fuel injection amount is corrected by increasingthe fuel injection amount when the air-fuel ratio of the exhaust gas iswithin a lean region, while the fuel injection amount set in the step ofsetting the fuel injection amount is corrected by reducing the fuelinjection amount when the air-fuel ratio of the exhaust gas is within arich region.
 16. The method as claimed in claim 9, wherein the exhaustgas purifying device includes a DPF, and the step of setting the fuelinjection amount includes a step of correcting the fuel injection amountset in the step of setting the fuel injection amount by reducing thefuel injection amount when it is judged that fuel needs to be suppliedinto the exhaust passage in the determining step during a regenerationprocess of the DPF.